How many DIY’ers have the capabilities to build a product as good as higher quality tube amps? Let’s say a Prima Luna power amp.
Depends on the DIY'er, how long they wish to spend with their build and so on. The hardest part of building an amplifier is connecting all the parts together--much easier with a board. Now it is very simple, use some software, design your board, "populate" it with the parts to make sure it all fits properly then send the board design out. Wait a week and you get your board in the mail. Of course, you already designed your amp (or whatever) tested it using software and you know what it will do before the first part ever touches the board. Thousands of people around the world do it every year, many videos on youtube to show you how to use the board, design and testing software and forums that discuss different designs.
Go to DIY Audio forum, they break down the amps into different types and you can play with tube amps, Class A, Class AB, Class D and Class H...even chip amps in parallel if you like.
Tube amps have been around for over 100 years--nothing mysterious about them at all. Class AB amps rolled out in the 1950's--nothing new under the sun their either--Class D was invented in 1964 so has been around for 55 years. It's just amplifiers, the dumb bricks of audio. Little squiggle goes in, bigger squiggle goes out--not hard to fathom. No chips, processors, software or anything exotic required to build them.
That Boulder thing--as stated in engineering classes the world over "Any fool can make something bigger--sometimes it takes a genous to make something smaller. Take a giant center tapped transformer, a huge bridge, slap in big storage/filter caps in the power supply and keep stacking NPN and PNP output transistors until you get the power and impedance drive levels you require. Be aware that more power and lower impedance requires larger heatsinks...and high bias Class AB requires huge heatsinks because they are not very efficient. Done!
OK, they took a regular amplifier and made it bigger--akin to making a huge toothpick out of a 2 by 4. The design is from the 1950's and they just kept adding parts until it reached specs. This is not hard to do, fairly simple with the hardest part being making the fancy case. Not what I would call an engineering feat in any shape or form. I own a Class AB amp that I call "The dumb brick", it just amplfies signals and nothing else. Even if it was 500 pounds, pushed 1/4 ohm loads and outputed 5,000 watts with my 220V dryer plug--it is still a dumb brick.
If you want to see actual engineering, at least something removed from mid last century--look at the professional arena amplifiers. Those things can programmable everything, every form of DSP, delay, dynamic EQ, speaker protection to programmable power supplies to tell it what breaker you have it attached to so it won't overload the breaker. Those things monitor in real time, get hammered to full power, can run outside in desert heat in Saudi Arabia for day long concerts, output 10 KW, do 1 or 2 ohm loads and weigh less than 30 pounds. They get flown or driven around the world on tour, don't break, don't blow up and do this for 2 year tours before they ever catch a break. Now THAT is some engineering--just throwing more parts/bigger parts and making a giant dumb brick is not impressive as far as engineering or moving things forward in amplifier technology.
If you want to know, just look at full testing of amplifier--it will tell you what it does. The best numbers I've seen were the Benchmarks, if you chase zeroes that would be the best bet. In reality, does it matter? Since my system is in a living room, not an underground bunker--I don't need 130dB S/N ratio specs. If I was in an underground bunker style studio where silence is golden, I would look into the Benchmarks for that reason. If I wanted/needed low impedance drive capability at high power levels, I'd look more towards professional touring amplifiers because they are designed to do that sort of thing without demainding me to rewire my house.
Now if you like massive oversized things just because that is your sense of style--or a way to compensate for something--have at it! I thought that giant amps were cool when I was a teenager but got over it during my PA days. Personally, I prefer smaller/lighter more efficient amplifiers as I tend to hide audio equipment--not make it the room center piece. YMMV, maybe you like giant things piled high in your living space--some people do! The market is mowing away from those so better get them while you still can!
In summation, you can build any amplifier you like--many people around the world do just that. Very easy to do these days with companies that wil make you the boards, software to design, test and refine your amp design and 3D printing to make it look like a space ship if you like--CNC machining can be done locally if you want something crazy. Back in the 40's/50's audiophiles used to build their own speakers and their own amplifiers--very common as that was part of the hobby. My ancestors did that, I remember seeing old tube amps, round face scopes, infinite baffle speakers with rheostats on the horns and all that sort of thing the audio clubs did in the 1950's. These days it would be far easier to build them than 65 to 75 years ago--the internet, communications and software make it much easier.
The DIY guys on AVS really don't get into building amplifiers--more speakers/subs and installing HT structural items but I think a few build amplifiers. You can get a bread board, purchase some components and build very basic amplifiers in an afternoon--fun hobby. There are youtube channels that will take you step-by-step, explain what each part is, what it does and why it is used. Give it a shot, might as well peak behind the curtain and see/learn how the magic works.